Third Eyelid

Pete Wedderburn
6th January 2020 - 3 min read

Humans and animals have many shared physical characteristics. We are told that over 98% of our genetic makeup is identical to animals, and as a result, there are far more similarities between the anatomy of humans and animals than there are differences.

beagle dog eye

Differences Between Animals And Humans

Most of the differences between animals and humans can be explained by the different lifestyles that we have. Cats have developed short, sharp barbs on the surface of their tongues, partly because they use their tongues so effectively in grooming their own coats. When a cat licks your hand, the rough feeling gives you a sense of how effective the tongue will be to remove loose hair from a bedraggled coat. Dogs and cats have large, pestle-and-mortar-like, grinding molar teeth at the back of the mouth because their food is coarser than our own. We can manage with smaller back teeth because we prepare our food more intricately before eating.  Humans no longer need the dense all-over fur coats of animals because we have learned how to create our own clothing.

The reasons for some of the differences are more difficult to understand. Humans have lost a small number of features that might have been useful to retain. We do not particularly want to be able to detect smells with the intensity of a dog. Nor would we find it useful to have long sensitive whiskers like a cat, so that we could judge the width of doorways. But some people would like to have retained the improved hearing of an animal. Clearer night vision would make it easier to spot cyclists without lights on those dark autumn evenings. And the rapid reflexes of a cat would be helpful in many aspects of modern life. 


Third Eyelid

The third eyelid is a physical structure that humans have lost, and again, it is hard to understand why it has gone. All animals, including humans, have upper and lower eyelids. Animals are blessed with an extra, third eyelid, that lies at the inner corner of each eye. It is a strong, rubbery, membrane-like structure. When an animal blinks, the third eyelid whizzes out and back across the surface of the eyeball, from side to side, like a curtain.  It provides an extra physical layer of protection for the eyeball, automatically shooting across the eye to protect it, as a part of the normal blink reflex. 

Visible Third Eyelid

In healthy animals, the third eyelid remains invisible, tucked neatly into the inner corner of each eye. Occasionally, especially in cats, illness can cause both third eyelids to become visible. Affected cats look peculiar, and their eyes look half-closed as if there is something seriously wrong. The problem seems to be caused by mild dehydration, or a low–grade viral infection. 

black cat eye

If humans had third eyelids, our eyes would be more robust, and we would be less prone to irritation from dust, pollens, and other airborne particles. There is nothing we can do about it, but it is interesting to wonder why they have gone.

It is also interesting to speculate about what else will disappear as humans continue to evolve. In the future, perhaps eyebrows will disappear from human faces. Fingernails might become increasingly smaller, and then vanish. Our descendants in ten thousand years may look startlingly different to ourselves.


  • Humans and animals each have unique anatomical features.
  • The “third eyelid” is present in most animals, but absent in humans.
  • Humans only have two eyelids: an upper one and a lower one.